DSoftStudio Mediator

Handler Validation

Overview

ValidateMediatorHandlers() is a source-generated extension method on IServiceProvider that detects misconfigured handlers at startup — before the first request is processed. It resolves every registered handler, pipeline chain, and notification handler from DI, and throws an AggregateException containing all failures if any handler cannot be resolved.

Usage

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddMediator(b =>
{
    b.AddOpenBehavior(typeof(LoggingBehavior<,>));
    b.AddRequestPreProcessor<ValidationPreProcessor>();
});

var app = builder.Build();

// Fail fast — detect missing handlers, broken dependencies, incomplete pipelines
app.Services.ValidateMediatorHandlers();

app.Run();

This works with both registration approaches — AddMediator(configure) and the step-by-step RegisterMediatorHandlers() + PrecompilePipelines(). The validation method operates on the built IServiceProvider, not on the registration API.

What It Validates

Handler type	Validation
Request handlers	`GetRequiredService<IRequestHandler<T,R>>()` + `GetService<PipelineChainHandler<T,R>>()`
Self-handling requests	`GetRequiredService<IRequestHandler<T,R>>()` (validates the generated adapter and its DI dependencies)
Notification handlers	`GetServices<INotificationHandler<T>>()` + materialization of all implementations
Stream handlers	`GetRequiredService<IStreamRequestHandler<T,R>>()` + `GetService<StreamPipelineChainHandler<T,R>>()`

When Validation Fails

System.AggregateException: One or more mediator handlers failed validation.
---> System.InvalidOperationException: No service for type 'IRequestHandler<CreateUser, Guid>'
---> System.InvalidOperationException: Unable to resolve service for type 'IUserRepository'
     while attempting to activate 'GetUserHandler'

Recommendation: Call ValidateMediatorHandlers() in development and staging environments. In production, consider gating it behind a configuration flag to avoid the startup cost of resolving every handler.

How It Works

The source generator emits ValidateMediatorHandlers() at compile time with explicit GetRequiredService<T>() calls for every handler it discovered. This means:

No reflection — no assembly scanning or type walking at runtime
No false positives — it validates exactly the same handlers the mediator will dispatch to
All failures reported at once — the method collects all resolution errors into a single AggregateException, so you see every problem in one pass